Channel fading is a fundamental characteristic of a wireless communication channel. Channel fading is the variability over time of the instantaneous conditions of the wireless communication channel. Channel-dependent adaptation of a transmission is one way to handle channel fading. When a channel is in a good state, it is possible to transmit data using a low level of error protection on the channel. In contrast, when the channel is in a bad fading state, a higher level of error protection is necessary, which causes a reduction in the information data rate. The level of error protection may, for example, be varied by changing the modulation format and channel coding rate. The information data rate is the number of transmitted information bits per second. In many wireless communication systems, a transmission is divided into blocks. It is possible to detect erroneous reception of a block, usually through a cyclic redundancy check (CRC). The ratio of incorrectly received blocks, either including or not including retransmitted blocks, is called the block-error rate (BLER).
In order to maximize the information data rate, the error protection level of the transmission should be adapted to the instantaneous channel conditions. This is often called “link adaptation.” In practice, link adaptation is often implemented by aiming at a fixed BLER (e.g., a target BLER).
In a typical wireless communication system many parameters of the communication channel used by a transmitting device to transmit data to a receiving device (a.k.a., the forward channel) can not be reliably estimated from parameters of the reverse channel (i.e., the channel used by the receiving device to transmit data to the transmitting device). In practice, this means that the receiving device may be able to directly determine the instantaneous channel conditions of the forward channel, but the transmitting device is not able to do so. Accordingly, in order for the transmitting device to adapt its transmission to the receiving device, the receiver needs to provide feedback (e.g., transmit) to the transmitter information indicating the quality of the channel. In a wireless communication system where the parameters of the forward communication channel can be reliably estimated directly at the transmitter, the channel quality can be directly computed at the transmitter, without the need of feedback.
Typically, a receiving device indicates the quality of a channel to the transmitter by transmitting to the transmitter one or more scalar quantities known as channel quality indicator (CQI) values, which indicate the quality of a channel for the purpose of describing the channel's ability to support information transfer. For example, a CQI value can indicate a recommended modulation format and code-rate based on channel measurements. For instance, the CQI value can be a signal to interference plus noise power ratio (SINR) value computed by the receiving device based on channel measurements. In a communication system with multiple parallel channels, there may be several CQI values, indicating the channel quality of each parallel channel or of subsets of parallel channels.
In a wireless communication system with CQI feedback, the CQI values are estimated (or predicted) at the receiver. Typically, the estimation involves the estimation of the instantaneous communication channel properties as well as computations based on a model of parts of the transmitter and a model of parts of the receiver. In practice, the channel estimation is noisy and the models of parts of the transmitter and the receiver may be inaccurate. Furthermore, for time-variant communication channels, the delay between channel estimation or prediction and the adaptation of the link induces additional inaccuracies in the CQI estimation. Therefore, it is common practice to obtain a back-off value and modify an estimated CQI using the back-off value, either at the receiver or at the transmitter or on both sides. Using a back-off value to modify a CQI value may increase the level of error protection compared to the originally computed CQI, but it may also decrease the level of error protection. For example, if the CQI is in the form of a modulation and coding scheme, the back-off may decrease the code-rate of the coding scheme, thereby increasing the level of error protection, compared to the originally computed modulation and coding scheme. The back-off value affects a computed CQI so that a larger back-off yields a higher level of error protection, and a smaller back-off yields a lower level of error protection. The computed CQI may be quantized so that a range of back-off values yields the same level of error protection. Note that the back-off value may be applied to any step in the computation of the CQI, not necessarily the final result. For example, if the CQI is a modulation and coding scheme, which is selected based on the computation of one or many SINRs, the back-off value may be applied to the one or many SINRs.
In a system with link adaptation and a target BLER, the back-off value may be adaptively adjusted over time in order to meet the target. One example implementation of such an adaptive back-off is that the receiver continuously estimates the BLER and changes the back-off with a term proportional to the difference between the estimated BLER and the target BLER. Another example implementation is that the back-off value is increased or decreased depending on whether the latest block was correctly received. By proper assignment of the increase step-size and the decrease step-size, the target BLER can be approximately met. Note that a back-off value may also be used in a system where the channel quality can be directly estimated at the transmitter.
Referring now to FIG. 1, FIG. 1 is a functional block diagram illustrating a link adaptation system having an adaptive back-off module. The adaptive back-off module compares an estimated performance indicator to a target performance indicator in order to decide how to adapt a back-off value. The back-off value is used by the link adaptation module in a process of selecting (e.g., computing or generating) a CQI value. Because performance of a link typically depends on the selected CQI, the link adaptation system implements a control loop that aims at meeting the performance target.
In wireless communication systems employing multiple-input-multiple-output (MIMO) technology, the communication link may often be represented by a plurality of links, here called “layers.” Each layer may have an individual channel quality or the layers may have the same channel quality. In many MIMO systems, it is possible to adapt the level of error protection on each layer separately. In such a system, it is beneficial to estimate and feed back CQIs, specific for each layer (or group of layers). In other MIMO systems, it is only possible to adapt the level of error protection on all layers simultaneously.
It is often beneficial to also adapt the number of layers that are used in the transmission, here called the “rank,” in addition to the level of error protection on each layer. Some aspects in the selection of the CQIs may depend on the selected rank. Hence, it may be advantageous to have a back-off value for each possible rank. The link adaptation system illustrated in FIG. 1 is still applicable to the case with adaptive rank-specific back-off, but with rank-specific performance target, performance estimate and back-off values, and a back-off adaptation box per rank. Note that the rank, as used here, is not necessarily related to the rank of a MIMO channel matrix.
In a MIMO system with rank-specific back-off values that are adaptively adjusted to meet one or many performance targets (e.g., a target BLER) it is reasonable to adjust the back-off for a rank only when the rank is used. If only one of a multitude of ranks is used for a period of time, it is natural to adjust the back-off for the used rank based on the measured performance during a period of time. It is often not feasible to adjust the back-off of a rank based on measurements concerning the transmission with other ranks.